Photographing TV and Computer Screen Images
From the brochure "KODAK Scientific Imaging Products", pages 26 and 27:
Photographing TV and Computer Screen Images
It is a relatively simple task to record a color or monochrome image
displayed on a television set, monitor, or computer screen. Although
a 35 mm single-lens-relfex (SLR) camera is recommended for ease of
focusing and framing the image, any automatic or adjustable camera will
yield good resutls. Close focusing capability is a must if the screen
image is to fill the film frame.
Several options are available: use a camera with a close-focusing
(macro) lens, use a lens with a fixed focal length of about 100 mm,
use "tele-extenders' to increase the focal length of a normal lens, or
use a supplementary close-up lens on a normal 50 mm lens. A lens of longer
focal length, (e.g., one of 100 mm), minimizes the effect of screen
curvature. You also need a stable tripod or platform because exposure
times are too long to permit hand-holding the camera.
Shutter Speed
Becuase images on a television or computer screen are formed line-by-line
by a rapidly moving electron beam, you can obtain a complete picture of the
screen only if the camera shutter speed is slow enough to allow the moving
beam to complete its scan. To avoid photographs with dark or light "banding,"
the beam should complete many scans.
TV images typically are composed of 525 horizontal scan lines (U.S.
standard) or 625 scan lines (the standard adopted by other countries).
For the U.S. standard, a complete scan takes 1/30 second (1/60th for the
odd- and 1/60 for the even-numbered lines). Theoretically you should use
a shutter speed no faster than 1/30 second, or 1/25 second for a 625-line
picture. These times are for a camera with a leaf shutter; for cameras
with a focal-plane shutter, reduce shutter speed to 1/8 second in order
to avoid getting a dark band across your picture and to stop TV action
scenes.
For computer screens, however, the subject is not moving and the
resolution requirements are much higher. Also, photographs are frequently
made of charts or graphs with light-colored backgrounds, and which show up
dark bands particularly well. When photographing computer screens, you can
obtain the best results with exposure times of 1/2 to 1 second. This exposure
puts many images of the screen on the film to minimize the effect of
"banding"--a film speed of ISO 100 is ideal.
Adjusting The Screen Image
Reduce the contrast of the screen image to slightly below the setting for
normal viewing. For a black-and-white image, adjust the brightness control so
that there is detail in the highlights and also in the shadows. For a color
image, adjust the color controls so the image is visually pleasing.
If your TV, monitor, or computer has an automatic brightness control--
one that varies image brightness in response to changing levels of room
lighting--turn the control off and adjust the brightness manually. For
computer screens, reduce the brightness so there are no scan lines visible on a
black screen.
Room Lighting
Darken the room completely if possible, or to a practical level to reduce
ambient illumination. Doing so helps make the area around the screen image
appear black in your pictures. A black surround is usually more pleasing than
a lighter one or one reflecting part of the room. If you photograph screen
images frequently, consider using a specially devised black "tent" or cone that
fits tightly around the screen.
Do not use flash or other lighting to illuminate the screen; it will
overpower the screen image. If you camera is equipped with automatic flash,
disable it or cover the flash with a piece of cardboard or other opaque material.
Film Recommendations
Use daylight color film to photgraph images on color screens. Color pictures of
color screen images may have a blue-green appearance because the sensitivity of
the film is different from that of the eye. Use a slow film, e.g., KODAK
EKTACHROME 100 Profressional Film. This film speed will call for a 1-second
exposure at f-8 to f-11. A 400-speed film may be too fast; you will not be
able to stop down to near f-11. Excellent photographs of computer screens are
made using small f-stops (to account for screen curvature) _and_ long shutter
speeds. KODACHROME 25 Professional Film has produced very good results.
To improve color rendition with KODACHROME and EKTACHROME Professional Films,
you may need to use a KODAK Color Compensating Filter CC10 or 20R over the
camera lens and to increase the exposure by 1/3 stop. Use a CC20B filter with
a color negative film such as KODAK VERICOLOR III Professional Film, which
provides brighter colors than and increased color contrast over the "conventional"
100-speed film. Experiment with this film, becasue it works better with some
phosphors than others.
You can also use the above color films to record the white, green, or amber
images from monochrome computer screens (without the use of color compensating
filters), or use black-and-white films if you do not need to retain the green or
amber colors in the picture.
[herein are listed a number of KODAK films. It is, after all, a KODAK document.]
Exposure Determination
Choose the correct shutter speed (1/2 to 1 second for computer screens and
1/4 to 1/8 second for TV images), and leave the f-stop stetting to be determined.
Consult your camera manual to determine how to set exposures with an essentially
fixed shutter speed.
Cameras equipped with a built-in exposure meter are useful, but the TV or
computer screen should fill the image area when the reading is taken. For
typical displays of "average" color and brightness content, you meter should
yield correctly exposed slides. For predominantly bright displays, bracket
exposures on the "overexpose" side; for predominantly dark screen displays,
bracket on the "underexposure" side. If you have through-the-lens metering
and you are using a color film with a color compensating filter over the camera
lens, your camera should automatically increase exposure to compensate for
attenuation by the filter.
To be sure of getting a properly exposed picture, bracket your exposures.
In 1/3-stop increments, overexpose or underexpose 1 full stop from any
calculated exposure.
Selected Reference
Eastman Kodak Company 1986. Photographing Television and Computer Screen
Images (AC-10).
To order publications from KODAK, write to:
Eastman Kodak Company
Dept 412-SP
343 State Street
Rochester, New York 14650-0608
Copied without permission, but then, it was a free catalogue, so I don't
think they'll mind.
Alan Berenbaum adb@research.att.com
AT&T Bell Labs
Murray Hill, NJ